For these reasons and more, a multiprobe engine monitor is a superb tool to help define day-to-day operating parameters, as well as long-term trends. Turbine operators routinely conduct what's called trend monitoring. By recording important parameters during every flight, you can compile a substantial database on the performance of your airframe and engine(s). With this backdrop, any changes in performance can be identified quickly.
Maintaining performance
Horsepower is a wonderful thing, providing the means and greatly influencing the quality of your airplane's performance. As a matter of course, you should keep a log of a few key performance figures so that you will have the background upon which any deviations will be obvious. Until someone invents an accurate, inexpensive power meter, this is the best way to know whether your 300-horsepower engine is really turning 300 hp.
Start with airspeed. Most performance numbers for aircraft that are gifted with a GAMA-style pilot's operating handbook are fairly realistic. Next trip, take a moment to double-check those figures.
To make the data as accurate as possible, calibrate the critical instruments. You can use a tach checker such as the Cardinal Electronics PropTach to see whether your tach is reading correctly. Calibrating your airspeed indicator involves flying two legs--one as directly upwind as you can, the other downwind--and averaging the two stabilized groundspeed numbers from your GPS. This gives you a calibrated true airspeed, which, when crunched for altitude and temperature, will tell you what your indicated should have been. (Incidentally, doing four legs 90 degrees apart in heading isn't accurate. Two of the four legs will always have some crosswind component, which lowers that leg's groundspeed and brings down the average.)
Begin a routine of jotting down a handful of important numbers during cruise flight. Items such as power settings, how the engine is leaned, fuel flow, indicated airspeed, altitude, outside-air temperature (cross-reference with the forecast winds and temperatures aloft for a gross error check), and aircraft load. You obviously don't have to massage the numbers in flight, but it will be valuable to track the performance statistics compared to the handbook, as well as over time for your aircraft.
What are you looking for? Any large, unexplained anomaly. If on one trip the airplane is unusually fast or slow compared to the book and its history, then you know something has changed. (Airplanes don't change performance just for the fun of it, although flight-test pilots might argue otherwise.) If it's fast, chances are that some instrument has drifted out of calibration. Is the fuel flow commensurately higher? Notice whether such a change is evident on the gauge or told by just the fuel truck. (Here's where keeping accurate track of fuel and oil consumption pays off.)
Whether your airplane has become faster or slower, begin a logical investigation to seek out the cause. This is where your historical database of performance is useful. Was it unusually hot outside this day? Did you fully compensate your power settings for nonstandard conditions? As a rule, to maintain power, nonturbo engines will need about 0.3 inches more manifold pressure for every 10 degrees Celsius over standard temperatures; subtract by the same amount for below-standard temps. With fixed-pitch models, you will need to add (or subtract) 25 rpm for every 10 degrees C above or (below) standard.
Engine parameters
Now that you've got your fancy new engine analyzer staring back at you from the panel, what do you do with the data? First you need a baseline. Fly your normal power and mixture settings, at your customary altitudes, to record the various temperatures and pressures. Notice how the CHTs stack up, and pay attention to the spread in EGTs during takeoff, climb, and cruise. At this point, noting the absolute EGT values is not important; you are just trying to understand the relationships. Engine analyzers that give you a digital readout of temperature in addition to the graphical presentation are the most useful for this task. (So, too, is data logging capability, which both the Insight GEM 610 and JPI EDM-700 can provide.)
There are times, though, when a large variation from the norms tells an interesting tale. Unusually high EGTs, for example, might come from a lean-running cylinder or might be the result of retarded ignition timing or a misfiring spark plug--the high EGT indicates that some of the combustion is taking place in the exhaust stack. A sticking exhaust valve that remains open during part of the power cycle will also net high single-cylinder EGT. Similarly, low EGT is an indication of an abnormally small combustion event. Trace this to an extreme fuel flow reduction (in effect, placing that cylinder well lean of peak); intake-air blockage; some valve train malady like a collapsed lifter, bent pushrod, or fractured cam lobe; or faulty ignition source.
Whatever you find, remember this: Start your troubleshooting with the simple, inexpensive items.
Lean-cycle tests
Using your multiprobe indicator to monitor a specified lean-cycle test will tell you a tremendous amount. For starters, it will disclose how well your engine's fuel/air ratios are balanced among the cylinders, and it will help to pinpoint induction leaks.
Set up the data-logging software for your engine analyzer; for the JPI this means connecting a laptop or palmtop computer to the data port; the GEM 610 will store the data internally. If you have another type analyzer that does not log data, make up a simple spreadsheet grid with spaces for fuel flow, EGT, CHT, airspeed, and oil temperature. You will be filling in the squares while conducting the lean cycle. (An alternative is to tape-record the event; but, so that you don't forget anything, have a cheat sheet of parameters to record.)
To prevent any chance of engine damage from overheating or detonation, set 65-percent power in cruise; both Continental and Lycoming permit leaning to peak at this power setting for most engine models. Because doing a lean-cycle test properly takes time and concentration, it's a good idea to have a safety pilot aboard to scan for traffic and hold out for smooth air and VFR conditions. Starting at full rich and watching the engine monitor, slowly lean the mixture. In lockstep, the EGTs and CHTs will climb and the fuel flow will drop. Watch for the first cylinder to reach peak EGT. Note which one it is and jot down the fuel flow. Continue to lean until the last cylinder has reached peak. In between, note at what fuel flow or at which cylinder's peak the engine becomes rough. Relatively few unmodified fuel-injected engines--and even fewer carbureted models--will run smoothly lean of peak EGT.
Most engines will display a predictable pattern, a certain cylinder that reaches peak first (usually dubbed the "lean" cylinder) and one or more that peak well after the others. Noting the fuel-flow spread between the first and last to peak will give you an idea of how well the fuel/air ratios are balanced. For a 250-hp engine, a spread of 1.5 gph is about average, with less than 0.7 gph being quite good.
Knowing which cylinder reaches peak first will tell you which one is the most critical for leaning and pinpoint the jug with the highest cruise CHT; cylinder temps hit maximum between 25 and 40 degrees Fahrenheit rich of peak EGT. It will be the lean cylinder, not the one with the highest absolute EGT at peak, that you should be using as a leaning reference. Current thinking suggests that maintaining all cylinders at least 35 degrees rich or lean of peak (50 degrees for turbos) will help to prolong valve and guide life.
Knowing the normal EGT and fuel/air ratio spreads will help you spot problems before they are evident on other instruments. For example, a cylinder that peaks very early suggests that its fuel/air ratio has gone lean. It is either getting less fuel--through a clogged fuel injector or a pinched delivery line--or more air. You can further pin this down by running the lean cycle at both full and partial throttle. An engine with even EGT peaks at full throttle but that is out of whack at partial throttle is steering you to a leaking induction coupler. This is true for nonturbo airplanes, but the reverse is likely to be true for turbo applications in which the normal cruise manifold pressure is well above ambient. In that case, a leaking coupling will make the affected cylinders run rich; depending upon where in the induction that leak is located, you could have one, two, or a whole bank of cylinders affected.
Many early fuel-injected installations used pressure gauges marked in flow; the accuracy of this gauge is totally dependent on the proper aggregate flow of the injectors. If one is blocked, the pressure and flow will indicate high, even though the total flow is in fact lower. A broken upper-deck reference line in a turbocharged airplane, on the other hand, will cause one cylinder to run rich, thereby lowering the indicated fuel flow at the normal peak turbine-inlet temperature. An engine analyzer will show that cylinder peaking late as well.
Fuel and oil consumption
Keeping good records of fuel and oil consumption will do more than make your tax accountant happy come April 15. Any short-term change in the consumption of either liquid should alert you to something amiss. Particularly where oil consumption is concerned, having a routine really helps. Make it a habit to check before every flight and note the level--at the same parking spot, if you can.
Engine manufacturers specify maximum oil consumption by a formula based on power output, but the high end is up near a quart an hour. There aren't many A who'd let an oil-burner like that through an annual, so a more realistic upper limit is closer to three to four hours a quart, with the optimum being around 10 to 12 hours a quart. Owners of big-bore Continentals who have seen virtually no oil consumption between changes have not, on the whole, experienced good top-end life.
Incidentally, owners who fail to use ongoing oil analysis and who do not insist upon opening the oil filter after each change are not using two of the most helpful indicators we have. Naturally, oil analysis can offer only indications of a trend. One or two samples don't offer much in the way of baselines because, in some subtle ways, each engine has its own signature. Peeking into the filter element each time will give you a read on the engine's natural effluvia, and often can disclose impending failures.
Differential compression checks
Here's another tool in the belt of the savvy owner, although it should be but one among many for determining your engine's condition--a single bad reading on a leakdown test is not necessarily reason to pull the jug. Find out first where the pressure is going--have you got a stuck or broken ring, a worn out valve guide, or just a piece of carbon under the intake-valve seat? Also, you Continental operators should make sure that the tester has the special orifice installed; otherwise, your readings will be suspect. Finally, always score compression readings against the engine's history. Has one cylinder been trending down over the last 100 hours? Or has one just taken a huge leap into the pond called top-end overhaul?
If all this sounds complicated and troublesome, well--nobody ever said that owning an airplane was easy. But armed with a keen sense of curiosity and an overflowing database of information about your airplane--its engine most of all--you can at least mitigate the financial sting and protect yourself from unnecessary maintenance diversions.
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